This invention relates to improved threaded connections for attaching together successive sections of an axially driveable pile or other similar driveable structure.
During the installation of an off-shore well drilling platform, it is customary to drive into the ocean floor a number of piles which function to connect the platform to the earth formation and locate and retain the platform in fixed position while transferring platform loads and forces to the soil Each pile is formed of a series of large diameter pipe sections connected together in end to end relation either by welding or by a specially formed mechanical connector. Since very heavy forces must be transmitted axially through such a pile as it is driven into the earth, the connections formed between successive sections of the pile must be of a character to withstand those heavy forces without adverse effect, and to maintain the integrity of the overall pile structure at all times.
Most of the mechanical pile connectors which have heretofore been devised have had very severe limitations in one or more respects, and as a result have not been used as widely as would be desired. Instead, welded joints continue to be utilized in the large majority of marine installations, even though welding requires a great deal of time to make a connection, and does not permit easy subsequent disconnection of the parts for removal and re-use.
One type of mechanical connector which has been devised incorporates a breech block structure in which the two parts are provided with circularly discontinuous thread-like teeth which are movable into and out of connected relation by relative rotation of the parts through a small angle. Such connectors unfortunately require extremely careful alignment of the pin and box members in order to move them into positions in which they can be joined by the limited relative rotation, and even after the connection is completed the joint does not provide a rigid connection between the parts and necessarily permits some slight relative movement between the pin and box members as a result of machining and assembly tolerances. That looseness between the pile sections can rapidly damage the connected elements under the loads imparted to the pile when struck by a hammer, and under the reverse rebounding loads to which the connection is subjected in the opposite direction. The connection is also relatively expensive to manufacture.
Another type of pile connector currently on the market employs a number of locking dogs which are carried by one pile section and are engageable with teeth formed on the other pile section in a camming relation intended to lock the two parts in rigidly fixed condition. This type of connector, however, is very difficult and expensive to manufacture, requiring the fabrication and handling of a large number of parts having very close tolerances and can break or release under the load forces encountered in use.
There have also been prior connectors which were intended to be driveable and which have included two parts attached together by interengaging threads. These joints have not, however, been capable of withstanding the pounding and rebounding forces to which a pile is subjected, and have tended to develop looseness in the joint causing its ultimate destruction. Additional driveable connectors devised in the past have employed interference fits of various types.
During the installation of an off-shore drilling platform, it is extremely important that the time period required for the overall installation procedure be kept to an absolute minimum. The equipment `spread` utilized for such platform installation, including derrick barges, tugboats, supply barges, etc., involves a very high daily cost, with rates of fifty-thousand dollars to five-hundred thousand dollars per day depending on location in the world and availability. In addition, this equipment can only work off-shore when the sea state is below a certain level. In some areas of the world this `weather window` or time of calm weather can be very short. The time required to properly set a platform, and the weather windows available at the site, are very carefully studied, and every possibility of reducing the time expended is explored. Since welding of a large size pile joint normally takes between eight and twenty-four hours, use of a connector which could do the job in minutes would greatly reduce the overall cost of installation of the platform and facilitate completion of the job within a limited weather window. It has also been found that piles can attain deeper and faster pernetration if the delay encountered during addition of a joint to the string is minimized.
The major purpose of the present invention is to provide a mechanical connection which can be utilized for attaching together successive sections of a pile or other axially driveable structure, and which makes a very rigid and permanently reliable connection between successive sections but does it more rapidly than by welding and more rapidly and effectively than by prior mechanical connectors devised for the same use. In addition to these advantages, a connection embodying the invention is structurally simpler and less expensive to manufacture than most previously devised driveable connectors with which I am familiar, and is more easily manipulated to make or break a joint.
A connection formed in accordance with the invention includes pin and box sections having engaging threads which are preferably of a unique type enabling transmission directly through the threads themselves of substantially all of the axial driving and rebounding forces to which the pile is subjected. In the previously proposed types of driveable connectors which have used threaded joints, the threads have not proven capable of taking such heavy forces, and as a result the pin and box sections are usually provided with transverse annular shoulders which abut against each other in the fully made-up condition of the joint to transmit axial forces through those shoulders. The area of engagement of these annular shoulders is however so limited that the shoulders tend to deform slightly and develop looseness under the driving forces, and in addition the shoulders of course can not assist at all in absorbing the reverse or rebounding forces which follow each impact of the driving hammer. Further, the shape of the threads has been such as to cause a camming action between the threads of the two parts inducing hoop stresses in the box member expanding it radially outwardly relative to the pin and thus further detracting from the strength and integrity of the joint.
To maximize the effectiveness with which both driving and rebounding forces are taken by the threads of the present invention, those threads are of a novel configuration having an overall axial taper in conjunction with an axial sectional thread form or shape in which the opposite side faces of the thread converge toward one another as they advance radially outwardly, but do so at a very small angle, and with each of those faces being disposed at an angle nearly but not quite perpendicular to the axis of the joint. More specifically, each of those opposite side faces of the thread as viewed in axial section should preferably be disposed at an angle of not more than about 5.5 degrees with respect to a line extending directly perpendicular to the axis of the threads, and desirably between about 4.5 and 5.5 degrees respect to that line. Each thread as viewed in axial section should also have a thickness between those opposite side faces and at the pitch line which is greater than the height of the thread between its root and crest, and preferably at least about one and one-half times that height.
At its crest, each thread should have a surface of substantial axial extent which tapers in accordance with the overall taper of the thread, and which preferably has a length in a direction parallel to the pitch line which is greater than the height of the thread perpendicular to that line. The threads are thus thicker axially than most threads to give them the strength required for taking axial forces, and have opposite side faces disposed sufficiently close to a directly transverse condition to take the axial forces in both directions directly through the threads without any substantial camming action tending to develop an excessive hoop stress in the box member or an excessive constricting force in the pin section.
The overall area defined by the engaging opposite side faces of these threads is much greater than can be attained in a conventional annular shoulder type engagement, and thus the joint as a whole can take axial forces in both directions very effectively. The joint is also rendered self-locking by the very slight angularity of the thread faces with respect to a line perpendicular to the axis of the joint, and by giving the thread a very small helix angle. This self-locking characteristic may be enhanced by forming the thread surfaces of a material having a relatively high coefficient of friction. Additionally, the taper of the threads in conjunction with their above discussed cross-sectional configuration and helix angle enables the pin and box to be stabbed together very easily without precision pre-aligning of the parts. The taper further supplements the self-locking characteristics of the joint when made up tightly to a predetermined torque, forcing the opposite side faces into tight engagement frictionally resisting unscrewing of the joint even under the heavy pounding forces which are encountered in driving of a pile
Other features of the invention relate to the provision of a unique type of positive locking system for mechanically preventing relative rotation of two threadedly connected parts from a predetermined fully made-up condition. To attain such a locking action, I may employ a locking element which is carried about one of the two threadedly interconnected sections, and is desirably retained thereon by extension more than half way about the part, preferably substantially entirely thereabout. This locking element has lugs which are receivable within notches formed in both of the two threaded sections to interconnect the sections against relative rotation. The locking element or ring may be movable relative to the section about which it is carried from a locking position to an inactive or retracted position in which it does not prevent relative rotation of the threaded parts. Fasteners may be provided for retaining the locking element in its active locking position.